1. Field of the Invention
The present invention relates to a projection display apparatus, and an information processing system and image recording/reproducing system using the same and, more particularly, to a so-called multiple-plate type projection display apparatus using a plurality of image display elements.
2. Related Background Art
Attention has recently been paid to a projection display apparatus for enlarging and projecting a display image on the image display element of a liquid crystal panel or the like. Along with this trend, demands have arisen for further improvements in the image quality and brightness of the display image and reduction in the size and weight of the whole apparatus.
Projection display apparatuses are classified into a single-panel type apparatus using one liquid crystal panel and a multiple-panel type apparatus using a plurality of liquid crystal panels. Many projection display apparatuses aiming high image quality use three panels corresponding to red, green, and blue light components. A projection display apparatus using multiple, e.g., three liquid crystal panels will be described.
The arrangement of a general multiple-plate type projection display apparatus will be described with reference to FIG. 24. FIG. 24 shows an overview of an optical system 101 in the projection display apparatus. The optical system 101 of the projection display apparatus is mainly constituted by an illumination optical system 110 and projection optical system 102. An image display element 105 (105a to 105c) made up of liquid crystal panels or the like is illuminated by the illumination optical system 110, and an image on the illuminated image display element 105 is formed on a screen 112 via the projection optical system 102.
In the illumination optical system 110, the image display element 105 is made up of the three, first, second, and third liquid crystal panels 105a, 105b, and 105c. These liquid crystal panels 105a, 105b, and 105c are transmission panels which are driven by an electric circuit (not shown) to display images to be projected.
The optical path and optical elements from the illumination optical system 110 to the screen 112 will be explained. In the illumination optical system 110, a light source section 109 has a light source 109a for emitting white light, and a reflector 109b for collimating a ray from the light source 109a. An integrator section 108 increases the uniformity of illumination light, and is a fly-eye integrator in this case. The integrator section 108 sometimes comprises, e.g., a polarization conversion element for increasing the use efficiency of illumination light.
In the illumination optical system 110, a color separation system 106 separates the optical path of white light from the light source 109a in units of wavelength regions, i.e., colors. For descriptive convenience, the color separation system 106 separates white light from the light source 109a into three representative wavelengths xcex1, xcex2, and xcex3. In practice, the respective optical paths correspond to red, green, and blue colors which respectively include the wavelengths xcex1, xcex2, and xcex3 which respectively indicate the center of wavelength of the transmitting light or at which the respective transmittances become maximum.
In the color separation system 106, a first dichroic mirror 106a has a function of transmitting only a light component with the wavelength xcex1 out of white light from the light source 109a, and reflecting light components with the wavelengths xcex2 and xcex3. The light component with the wavelength xcex1 having passed through the first dichroic mirror 106a is deflected by a deflection means 107 to illuminate the first liquid crystal panel 105a via a lens. The light components with the wavelengths xcex2 and xcex3 reflected by the first dichroic mirror 106a are incident on a second dichroic mirror 106b. 
The second dichroic mirror 106b reflects the light component with the wavelength xcex2, and transmits the light component with the wavelength xcex3. The light component with the wavelength xcex2 reflected by the second dichroic mirror 106b illuminates the second liquid crystal panel 105b via a lens. The light component with the wavelength xcex3 having passed through the second dichroic mirror 106b illuminates the third liquid crystal panel 105c via the deflection means 107 and a relay system 111 including a plurality of lenses.
The projection optical system 102 comprises a cross prism (cross dichroic prism) 104 for color combination. The above-described transmission liquid crystal panels 105a to 105c are arranged near respective incident surfaces of the cross prism 104. With this arrangement, the projection optical system 102 combines the optical paths of the light components with the wavelengths xcex1, xcex2, and xcex3. A projection lens 103 is arranged near the exit surface of the cross prism 104. Images on the illuminated liquid crystal panels 105a, 105b, and 105c are projected on the screen 112 so as to overlap each other.
In the projection display apparatus having this arrangement, the illumination light components of the first, second, and third liquid crystal panels 105a, 105b, and 105c are combined as an image by the cross prism 104, projected via the projection lens 103, and displayed on the screen 112 so as to overlap each other. In the projection display apparatus, therefore, pixels constituting the liquid crystal panels 105a to 105c must overlap each other, and aberrations of the optical system 101, particularly chromatic aberration of magnification of the projection optical system 102 needs to be further reduced.
As for reduction in chromatic aberration, techniques of greatly reducing chromatic aberration by arranging a diffraction optical element in a refraction optical system are conventionally disclosed in Japanese Patent Application Laid-Open No. 06-194509 xe2x80x9cOptical System Including Diffraction Optical Element, and Diffraction Optical Elementxe2x80x9d, Japanese Patent Application Laid-Open No. 08-043767 xe2x80x9cImage Sensing Optical Systemxe2x80x9d, Japanese Patent Application Laid-Open No. 10-213744, and the like.
FIGS. 25A and 25B show the sections of optical systems disclosed in Japanese Patent Application Laid-Open Nos. 08-043767 and 10-213744, respectively. FIG. 25A shows an image sensing optical system for a telescope lens or the like. A surface r3 of a parallel plate located closest to the object side is formed as the formation surface of a diffraction optical element to greatly reduce chromatic aberration without using any low or extra-low dispersion glass.
As shown in FIG. 25B, Japanese Patent Application Laid-Open No. 10-213744 discloses an optical system for a finite-distance zoom lens made up of a lens unit GR1 having negative and positive meniscus lenses, and a lens unit GR2 having a biconvex positive lens, stop (A), and negative and positive meniscus lenses. Diffraction optical elements are formed on surfaces (HOE) in FIG. 25B, i.e., a convex surface r3* of the positive meniscus lens of the lens unit GR1 and a convex surface r11* of the positive meniscus lens of the lens unit GR2. This realizes a small number of lenses in the lens units GR1 and GR2, a small-size zoom lens, and high performance.
Reduction in chromatic aberration is achieved in this case because the diffraction optical element has characteristics opposite to the dispersion of an optical glass having xcexdd=about xe2x88x923.453 in Abbe constant, and has high dispersion characteristics. The diffraction optical element is very thin because the grating structure exhibits these characteristics, and can downsize the optical system.
The diffraction optical element has these properties, can greatly improve optical performance, but must consider parasitic-diffracted light (stray light) generated by diffraction.
Parasitic-diffracted light (stray light) and diffraction will be explained by exemplifying a case wherein the incident angle to a diffraction grating is 0xc2x0, as shown in FIG. 26. Letting P be the grating interval of a diffraction grating, xcex be the wavelength, and m be the diffraction order, an angle e at which mth-order diffracted light is deflected satisfies
mxc2x7xcex=p sin xcex8xe2x80x83xe2x80x83(1) 
When the diffraction efficiency is designed to be 100% at a wavelength xcexo, the diffraction efficiency xcex7m for light having the wavelength xcex and mth-order is given by
xcex7m=sin c2(xcexo/xcexxe2x88x92m)xe2x80x83xe2x80x83(2) 
From equation (2), the diffraction efficiency depends on the wavelength. For example, for
xcex=xcexo and m=1
in equation (2), the diffraction efficiency is 100%. At other wavelengths, however, the diffraction efficiency for m=1 does not become 100%, and parasitic-diffracted light (stray light) is generated.
Characteristic (2) shown in FIG. 27B shows the wavelength dependence of the diffraction efficiency when the wavelength reaches 530 nm at the peak of the diffraction efficiency of a design order. As is apparent from FIG. 27B, the diffraction efficiency is 100% around a wavelength of 530 nm at the peak of the designed order, but greatly decreases at 400 nm or 700 nm.
The decrease in diffraction efficiency at the design order acts as parasitic-diffracted light (stray light), and the light is diffracted in a different direction from the design order in accordance with equation (1). For this reason, the prior art suffers image degradation caused by parasitic-diffracted light (stray light) in picking up an image with a light source having a wide wavelength region such as a visible light region having a high luminance.
As for a wavelength at which the diffraction efficiency maximizes, when the design order is the 1st order, the design wavelength is xcexo, d represents the grating height of the diffraction optical element, No represents the refractive index of the structure of the diffraction optical element having the wavelength xcexo, and the refractive index of air is 1, as shown in FIG. 26, and:
xcexo=d(Noxe2x88x921)xe2x80x83xe2x80x83(3) 
holds, the diffraction efficiency for the wavelength xcexo is 100%. At wavelengths other than xcexo, equation (3) does not hold, and the wavelength dependence of the diffraction efficiency appears.
Japanese Patent Application Laid-Open No. 08-220482 discloses a technique of reducing image degradation caused by parasitic-diffracted light (stray light) of the diffraction optical element used in a wide wavelength region like a visible light region or a plurality of wavelength regions. This technique will be described with reference to FIGS. 27A to 27C.
FIG. 27A shows the schematic section of an optical system. This optical system illuminates an object 2 by an illumination optical system 1, and forms the image on an image plane 4 via a projection optical system 3. The illumination optical system 1 for illuminating the object 2 comprises, e.g., a light source 5 for emitting visible white light, collector lens 6, aperture stop 7, condenser lens 8, and segmented wavelength selection element 9. The projection optical system 3 for projecting the image of the object 2 comprises lens units 12 and 13, and a diffraction optical element 11 having a relief pattern surface 10 on which a concentric relief pattern is formed.
The diffraction optical element 11 is on the pupil of the projection optical system 3, and the relief pattern surface 10 is divided into regions, as shown in FIG. 27C. Regions A, B, and C of the diffraction optical element 11 in FIG. 27C have diffraction efficiencies corresponding to characteristics (1), (2) and (3) in FIG. 27B, respectively. With this setting, uniform diffraction efficiencies can be obtained in the entire region of the diffraction grating 10.
In this optical system, assume that, e.g., the diffraction efficiency of region C corresponds to characteristic (3) in FIG. 27B. Since the diffraction efficiency around 400 nm is very low, light in a wide wavelength region incident on the entire region on the pupil generates parasitic-diffracted light (stray light).
One of methods of reducing the wavelength dependence of the diffraction efficiency of the diffraction optical element is a technique disclosed in Japanese Patent Application Laid-Open No. 09-127321. FIG. 28 shows its schematic structure. A diffraction grating having a plurality of stacked optical materials is constituted as shown in FIG. 28, thereby reducing the wavelength dependence of the diffraction efficiency. In this case, however, the grating structure is complicated.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a projection display apparatus having a projection optical system which achieves high performance and small size and reduces parasitic-diffracted light (stray light) even with the use of a diffraction optical element having a simple arrangement, and an information processing system and image recording/reproducing system using the projection display apparatus.
To achieve the above object, according to one aspect of the present invention, there is provided a projection display apparatus comprising color separation means for separating light from a light source into a plurality of beams having different wavelength regions, a plurality of image display elements each inserted in each of optical paths of the beams separated by the color separation means so as to be illuminated by the beams, color combining means for combining the beams emerging from the plurality of image display elements, and projection means for projecting the beams combined by the color combining means on a projection surface, wherein a diffraction optical element for reducing chromatic aberration of magnification of the projection means is inserted in at least one of a plurality of optical paths between the plurality of image display elements and the color combining means.
The diffraction optical element is very thin, and is hardly spatially limited. Arranging the diffraction optical element in the optical system can greatly reduce aberrations, particularly chromatic aberration of magnification while decreasing the number of lenses.
The diffraction optical element, which is inserted in the optical path after color separation, can individually correct aberrations within only a given spectral region to remarkably improve the imaging performance. By limiting aberration correction to a given spectral region, parasitic-diffracted light (stray light) can be reduced.
In the projection display apparatus, letting xcexc be a central wavelength of a wavelength region of a beam illuminating an image display element on an optical path in which the diffraction optical element is inserted, or a wavelength having the highest luminous intensity in the wavelength region, and xcexi be a wavelength having the highest diffraction efficiency of the diffraction optical element, the wavelengths xcexc and xcexi satisfy
0.0 less than ABS((xcexixe2x88x92xcexc)/xcexc) less than 0.14 
where ABS represents the absolute value of a numerical value within the parentheses, xcexc is a wavelength falling within a visible light wavelength region of 400 nm to 700 nm, and xcexi is a wavelength falling within a wavelength region of 400 nm to 500 nm, 500 to 600 nm, or 600 nm to 70 nm.
In further aspect of the projection display apparatus according to the invention, the diffraction optical element includes a transmission-type phase grating.
In further aspect of the projection display apparatus according to the invention, each image display element includes a transmission-type liquid crystal element.
In further aspect of the projection display apparatus according to the invention, a sectional shape of said diffraction optical element is a kinoform shape or a stepwise shape that has not less than eight steps and approximates the kinoform shape.
In further aspect of the projection display apparatus according to the invention, the color separation means separates light from the light source into beams having red, green, and blue wavelength regions.
In further aspect of the projection display apparatus according to the invention, the diffraction optical element is inserted in an optical path of the beam in the blue wavelength region.
In further aspect of the projection display apparatus according to the invention, diffraction optical elements are inserted in at least two of the optical paths between said plurality of image display elements and said color combining means.
In further aspect of the projection display apparatus according to the invention, said diffraction optical elements are designed to exhibit the maximum diffraction efficiency at different wavelengths.
In further aspect of the projection display apparatus according to the invention, the diffraction optical element is inserted in each of optical paths between said plurality of image display elements and said color combining means.
In further aspect of the projection display apparatus according to the invention, said color combining means includes a dichroic prism which has at least two light incidence surfaces, and combines beams having different wavelength regions from the respective incidence surfaces to output the combined beam.
In further aspect of the projection display apparatus according to the invention, said color combining means comprises dichroic mirrors for transmitting, reflecting, and combining the beams having the different wavelength regions.
According to another aspect of the invention, there is provided an information processing system which comprises the projection display apparatus mentioned above and a computer for generating image information to be supplied to said projection display apparatus.
According to another aspect of the invention, there is provided an image recording/reproducing system which comprises the projection display apparatus as mentioned above and an image recording/reproducing device connected to said projection display apparatus to record and/or reproduce image information to be supplied to said projection display apparatus.
According to another aspect of the invention, there is provided a projection display apparatus which comprises color separation means for separating light from a light source into a plurality of beams having different wavelength regions, a plurality of image display elements each inserted in each optical paths of the beams separated by said color separation means so as to be illuminated by the beams, color combining means for combining the beams emerging from said plurality of image display elements and projection means for projecting the beams combined by said color combining means on a projection surface, wherein a diffraction optical element is inserted in at least one of a plurality of optical paths between said plurality of image display elements and said color combining means.